The present invention relates to ignitors for explosives and gas generators, and in particular to ignitors that incorporate electrostatic discharge features.
Ignitors for explosives and gas generators present a dichotomy between the desire for reliably consistent ignition and the importance of avoiding unintended initiation. The approach to solving this dichotomy is to reduce the sensitivity of the ignition charge and to increase the power or unique characteristics of the ignition signal. Despite the advances that have been made, static discharge is still a concern, because of the possibility that a static charge could discharge through the ignitor and cause an unintended initiation.
Known solutions to the problem of electrostatic discharge include making provision for an electrostatic charge to be grounded to the ignitor case, thus preventing static current from flowing through the heating element. Such solutions typically require additional costs associated with fabricating the required static discharge structure. U.S. Pat. No. 2,408,125 describes a means for safeguarding electric ignitors of blasting detonators against accidental firing due to static electricity. The means comprises teeth, projections, edges or other means adapted to constitute discharge points which promote or facilitate the discharge of high-voltage from the means into a grounded mass, wherein the grounded mass consists of the material which resists low voltage currents but readily passes high-voltage currents. U.S. Pat. No. 2,408,125 suggests materials such as Galena, yellow crystalline iron pyrites, or zincite. U.S. Pat. Nos. 5,433,147 and 4,261,263 disclose the use of a spark gap in a detonator to dissipate static electricity. All prior art structures for discharging static electricity require the addition of significant structure, or modifications of previously existing designs.
What is needed is a static discharge mechanism that adds little or no cost, and requires minimal modifications to modern airbag ignitors.
The ignitor of this invention has a circuit board mounted within a metal can. Mounted on the circuit board are various electronic components used for initiating a semiconductor bridge that functions as a. The heating member ignites a reactive material, which in turn ignites a pyrotechnic material such as an explosive or a gas generating compound. The explosive may be used in mining, construction or demolition operations. The gas generated by a gas generating compound may be used for inflating an airbag in a vehicle or powering a seatbelt pretensioner or other vehicle safety device. The circuit board has a copper surface that is etched to form the copper traces used to connect circuit components. A mask used to form the copper traces is modified to form a static discharge point electrically connected to each of the incoming electrically conducting leads. The sharp two-dimensional trace has a triangular point that concentrates lines of flux. The etched point is spaced between about 0.076 mm to about 0.127 mm (about 0.003 to about 0.005 inch) from the grounded case, or a corresponding triangular point that is conductively connected to the grounded case, so as to form a potential gap which readily discharges static electricity but does not ground ordinary currents flowing through the wire connected to the discharge points.
Because the discharge points only require a change in the mask used to fabricate the traces on the circuit board, nothing is added in the way of processing steps or material to the manufacture of the ignitor. The only change is to remove less copper in the etching solution, reducing costs.
The present invention provides a means for static discharge in an ignitor which does not add cost to the production of the ignitor.
It is a further feature of the present invention to provide an ignitor which is less subject to unintended initiation caused by static electrical discharge.
The present invention further provides a method of adding an electrostatic discharge mechanism to an ignitor that does not require any additional process steps in the manufacture of the ignitor.
Further features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.